1. Field of the Invention
This invention relates to gas turbine engines, and more particularly to the support of stator vanes in such an engine.
2. Description of the Prior Art
A gas turbine engine has a compressor section and a turbine section and includes a rotor extending axially through the compressor section. A row of rotor blades extends outwardly from the rotor. A stator circumscribes the rotor. The stator includes an outer case and an array of stator vanes extending inwardly from the outer case. A gas stream flows axially through alternate rows of rotor blades and arrays of stator vanes. The rotor blades of the turbine extract energy from the gas stream to drive the rotor blades of the compressor. Commonly, the vanes of each array receive the gas stream from the upstream row of stator blades and direct the gas stream to a downstream row of rotor blades. For successful operation, the rows of rotor blades and the arrays of stator vanes must be concentrically and radially aligned. Concentric alignment of the array of stator vanes is provided by support structure extending inwardly from the outer case.
In one typical engine structure, the stator vanes of each stator array are cantilevered inwardly from support structure attached to the outer case of the stator. U.S. Pat. No. 3,066,911 to Anderson et al. entitled "Nozzle and Turbine Wheel Shroud Support" is representative of cantilevered support structures. In Anderson an outer shroud ring provides support to cantilevered stator vanes and an unsupported inner ring joins together the inner ends of the stator vanes. The gas stream loads on each vane are taken out through the outer end of the vane. This end must resist all of the axial loads and bending moments on that vane.
In another common engine structure the vanes are simply supported between an inner support and an outer support. U.S. Pat. No. 2,968,467 to McGregor entitled "Connecting Means, Especially For Securing Annular Stator Elements Between Supports Whose Positions Are Fixed" shows a representative simply supported vane. The outer support restrains the vane axially and radially and the inner support restrains the vane axially. The gas path loads on each vane are taken out through both ends of the vane. The inner end and the outer end together resist the axial loads and the bending moments on that vane.
Simply supported vane systems are not without problems. Differences in thermal growth between the inner support and the outer support create axial and radial stresses. The support structure in McGregor and other simply supported structures such as U.S. Pat. No. 3,062,499 to Peterson entitled "Vane Mounting and Seal" share this problem. Differences in axial growth between the inner and outer supports subject the stator vane to cyclic stresses and eventual low cycle fatigue failure. In McGregor, the outer support is fastened to the outer case. Thermal excursions of the case cause misalignment between the stator array and the blades of the rotor. In addition to the misalignment problem the distortions and stresses may be severe enough to impair sealing at the inner and outer supports. A loss in engine efficiency and durability results. Thus, even though simply supported vane arrays are stronger and safer than cantilevered arrays, thermal growth problems remain to be solved.
The need to produce energy efficient machines has grown in recent years because of increased fuel costs and limited fuel supplies. Because of the twin needs of economy and safety, research efforts are being directed at decreasing the stresses in arrays of stator vanes and at keeping the vane arrays in alignment with the blades of the rotor.